1. Field of the Invention
This invention relates to a frequency adjustment circuit, specifically to a frequency adjustment circuit to adjust a frequency of an oscillation circuit that oscillates according to a time constant set by a time constant circuit.
2. Description of the Related Art
System clocks of an LSI (Large Scale Integration) are generated based on clocks generated from an RC oscillation circuit that uses a resistor R and a capacitor C, and is incorporated in the LSI. When the resistor R and the capacitor C are incorporated in the LSI, however, the oscillation frequency varies by the oscillator circuit because variations are caused in characteristics of the passive devices due to variations in a manufacturing process of the LSI. Therefore, the oscillation frequency has been adjusted to a target value using frequency adjustment data produced using a zapping device such as a polysilicon fuse.
FIG. 16 is a circuit diagram showing a frequency adjustment circuit according to a prior art. An RC oscillation circuit 10 is provided with an oscillation loop composed of a hysteresis inverter 11, an inverter 12 and a P-channel type MOS transistor 13, a waveform shaping circuit 14 that shapes an oscillation wave from the oscillation loop and outputs clocks OSCCLK and a time constant circuit 15 connected to the oscillation loop and composed of resistors Rosc1, Rosc2, Rosc3 and Rosc4 and a capacitor Cosc.
Each of a first zapping circuit 21 and a second zapping circuit 22 generates each of frequency adjustment data ZP1 and ZP2 that correspond to connection/disconnection of zapping devices, respectively, according to a reset signal RESET. Connection/disconnection of each of the zapping devices is permanently set according to voltages applied to zapping terminals ZAP1 and ZAP2.
The frequency adjustment data ZP1 and ZP2 generated from the first zapping circuit 21 and the second zapping circuit 22 is decoded with a frequency adjustment data decoder 23. Each of decoded data ZDC10, ZDC00 and ZDC01 is applied to each of ON/OFF control terminals of switches SW1, SW2 and SW3 made of CMOS analog switches, respectively.
For example, when the frequency adjustment data (ZP1, ZP2) from the first zapping circuit 21 and the second zapping circuit 22 is (1, 0), the decoded data (ZDC10, ZDC00, ZDC01) from the frequency adjustment data decoder 23 is (1, 0, 0). Since the switch SW1 is turned on while the switches SW2 and SW3 are turned off as a result, one end of the resistor Rosc1 is grounded. This makes the time constant circuit 15 being composed of the resistor Rosc1 and the capacitor Cosc. In this case, the RC oscillation circuit 10 oscillates at a frequency fosc that corresponds to the time constant determined by the resistor Rosc1 and the capacitor Cosc. That is, when the P-channel type MOS transistor 13 is turned on, the capacitor Cosc is charged and an electric potential at an input terminal of the hysteresis inverter 11 is raised. When an output of the hysteresis inverter 11 is reversed, the P-channel type MOS transistor 13 is turned off through the inverter 12.
As a result, electric charges stored in the capacitor Cosc are discharged to the ground through the resistor Rosc1. Then the electric potential at the input terminal of the hysteresis inverter 11 is lowered. When the output of the hysteresis inverter 11 is inverted once again, the P-channel type MOS transistor 13 is turned on through the inverter 12. The RC oscillation circuit 10 oscillates by alternating the charging and the discharging. Therefore, according to the frequency adjustment circuit, the oscillation frequency of the RC oscillation circuit 10 can be adjusted by generating desired frequency adjustment data ZP1 and ZP2 according to the connection/disconnection of the zapping devices.
Further information on the technologies described above is disclosed in Japanese Patent Application Publication No. 2000-148064, for example.
However, when an external noise is applied to a certain terminal and enters into an inside of the LSI incorporating the RC oscillation circuit 10 described above while the LSI is in operation, the frequency adjustment data retained in the first zapping circuit 21 and the second zapping circuit 22 is changed in some cases. For example, when the frequency adjustment data ZP1 of the first zapping circuit 21 is changed from “1” to “0”, the frequency adjustment data (ZP1, ZP2) is modified to (0, 0). Corresponding to the modification described above, it is assumed that the decoded data (ZDC10, ZDC00, ZDC01) from the frequency adjustment decoder 23 is modified from (1, 0, 0) to (0, 1, 0).
Then the resistor and the capacitor constituting the oscillation loop are modified from the resistor Rosc1 and the capacitor Cosc to the resistors Rosc1 and Rosc2 and the capacitor Cosc. The modification results in an oscillation frequency lower than the target frequency fosc. In order to recover the frequency adjustment data (ZP1, ZP2) changed by the external noise to the normal data (1, 0), inputting the reset signal RESET once again is required. However, recovering the frequency adjustment data (ZP1, ZP2) to the normal data has been practically impossible, because the reset signal has been usually designed to be inputted only at power-on of the LSI.